1. Field of the Invention
The present invention generally relates to externally pressurized gas bearings.
2. Description of Prior Art
Externally pressurized gas bearings and the use of gas as a lubricant is widely known. The major advantages of gas lubrication over liquid lubrication are well understood. Due to the low viscosity of the gas lubricant, gas bearings have very low coefficients of friction. The stability of gas as lubricant, allows gas bearings to operate in both high and low temperature environments where liquids would solidify, vaporize, cavitate, or decompose. Gas lubricants frequently eliminate the need for bearing seals, particularly when air is the lubricant and the atmosphere is the reservoir. Gas bearings operate with very small clearances and thus maintain close tolerances.
The disadvantages of gas bearings are also recognized. Prior art teaches that because of the low viscosity and low damping in gas films, gas bearings have a reduced unit load carrying capacity. Generally gas bearings are larger and operate with very thin films compared to their liquid lubricated counterparts. The thin films in gas bearings demand very close control of machining tolerances and surface finishes.
The unit load capacity of gas bearings is pressure dependant. High unit loads require high gas pressures. Prior art teaches that high pressures produce either excessive gas flow or shock waves within the bearing. Hence gas bearings are thought to be incapable of operating at unit loads achievable by liquid lubricated bearings. Prior art has concentrated on avoiding shock waves by reducing pressures so as to maintain the flow in the lower subsonic velocity range which is adequately modeled by what is known as Viscous flow theory. Prior art teaches the desirability of maintaining low velocity gas flow within the bearing. There are many analytical and experimental studies on the phenomena of supersonic pressure depression in the feeding region of externally pressurized gas bearings. The deductive conclusion of this teaching is that raising the inlet pressure on a particular gas bearing has a negative effect on load capacity. However, in many such cases the supersonic flow and shock surfaces arise from the rapid expansion of the flow cross sectional area: That is; the geometry of the bearing inherently creates an area change dominated flow pattern.
Heretofore, prior art has failed to appreciate the advantages of controlling the geometrically induced flow cross sectional area expansion within a gas bearing. By controlling the flow cross sectional area within a bearing it is possible to have both high pressure and low flow rates. It is also possible to have gas velocities in the near sonic range producing large velocity pressure effects within the bearing.
Prior art includes a number of considerations of compressible flow effects which are relevant to a specific application. These include air hammer instability and lock up which are sometimes related to each other. Lock up can occur when the gas supply pressure multiplied by the area of the recess is less than the load. Avoiding lock up is crucial in many situations such as opposed pad thrust bearings and hydrostatic journal bearings.
The various theoretical models used in simple compressible gas flow are useful in designing and understanding high load gas bearings. The three most important factors altering the state of a flow stream are changes in area, wall friction, and the addition or removal of heat. Each effect has an associated simple theoretical model. Respectively they -are simple area change, simple friction, and stagnation temperature change. For a compressible gas these flow models are also known respectively as Isentropic flow, Fanno line flow, and Rayleigh line flow.
With respect to gas bearings the simple characteristics of these flow types can lead to great insight into how to optimize high pressure gas bearing design from a qualitative point of view.
Area change effects such as those which occur in converging and diverging nozzles are often the subject Isentropic gas flow analysis. Many elementary gas analysis deal with adiabatic supersonic and sub sonic flows within nozzles. This type of analysis can lead to the recognition that in gas bearings, area change effects are often the cause of shock waves. Such shock waves, as previously mentioned are generally deemed undesirable in gas bearings. Prior art has concentrated on avoiding such shock waves by reducing pressure and hence unit load capacity.
The small clearances within gas bearing often results in length to equivalent diameter ratio on the order of several hundred or more making friction a significant factor. Specifically designing a bearing to be friction dominated can be very desirable. In such a bearing, a Fanno line flow gas bearing, the gas enters the clearance space at a subsonic velocity and exits at a higher velocity, often near Mach 1.0, under the nominal design conditions. Thus, the gas velocity gradually rises as the gas proceeds across the bearing surface. The entering mach number can be estimated by complicated but workable methods once it is known that the pressure supplied is sufficient to force a shock exit. Or conversely, one can assume a shock exit and back calculate the entrance and exit conditions for a given entrance pressure. A key parameter of the physical system is known in the literature as 4fL/d where f is friction factor, L is a length, and d is the equivalent diameter of the flow path. For clearance space of thickness h, d would often be taken as approximately two times h. FIG. 2 shows a plot of the flow path length across a bearing land area verses Mach number for a typical gas bearing with a mach 1.0 exit. FIG. 3 shows a plot of flow path length verses the stagnation pressure ratio for a typical fanno flow gas bearing with constant flow cross sectional area. Such procedures are well known in gas dynamics and are useful in designing Fanno flow bearings.
Similarly, it can be appreciated the Rayleigh line flow models in combination with Fanno line flows are informative in understanding normal shock in constant area flows. In certain situations involving high heat transfer, such as combustion, the Rayleigh line flow models can dominate. Often in gas bearings this effect is modest and can be incorporated as a pertibation to the dominate flow solution. Qualitatively, the effects of Rayleigh line flow are similar to Fanno flow in that within a constant area sub sonic flow the addition of heat along the flow channel increases the velocity to a maximum of Mach one at the exit.
High pressure bearings have small clearances and are often limited by the expense of manufacturing surfaces of the required tolerances. The use of compliant bearing surfaces can greatly alleviate the necessity for expensive machining while providing close tolerances. However, the advantages of compliant surfaces in controlling the character of the flow within the bearing are not appreciated by prior art.
Another important consideration that establishes the lower limit on clearance space for gas bearing is the mean free path length which is measure of the minimum characteristic distance required for a gas to be behave as a continuum. The mean free path of air at typical supply pressures is less than a 0.000001 inch so that even at nominal clearances of 0.000001 inch the gas flows as a continuum. It is thus practical to operate fanno flow bearings at pressures of several hundred pounds per square inch or more.
It will be appreciated that Fanno line flow gas bearings offer significant increases in bearing load capacity and dynamic response over conventional gas bearings.